This application is based on, and claims priority from, Japanese Patent Application No. 2012-218012, filed on Sep. 28, 2012 with the Japan Patent Office, the entire contents of which are incorporated herein by reference.
The present disclosure relates to a positively-charged single-layer electrophotographic photoreceptor and an image forming apparatus comprising the positively-charged single-layer electrophotographic photoreceptor as an image carrier.
Conventionally, organic photo conductors (OPCs) are widely used as photoreceptors in image forming apparatuses. Organic photo conductors can be roughly divided into single-layer organic photo conductors in which a single layer created by dispersing a charge generating material (CGM) and a charge transporting material (CTM) in a binder resin is formed on a tubular photosensitive layer support base made of aluminum or the like, and organic photo conductors in which a layer containing a CGM and a layer containing a CTM are laminated on a tubular photosensitive layer support base.
Among organic photo conductors, single-layer organic photo conductors have a simple layer construction and therefore offer superior productivity. In addition, when such a single-layer organic photo conductor is combined with a charging member which adopts a contact-charging system and used as a positively-charged single-layer organic photoreceptor, oxidized gas such as ozone which adversely affects office environment is hardly generated.
Therefore, due to such advantages, positively-charged single-layer electrophotographic photoreceptors are becoming more utilized.
An electrophotographic photoreceptor is manufactured by applying the photosensitive material on a circumferential surface of a photoreceptor support base.
In addition, an application method thereof usually involves moving a container (a coating tank) that houses an application liquid of the photoreceptor material and the support base relative to each other, dipping the support base in the application liquid, and pulling the support base out from the container at a predetermined speed.
According to the adopted method, the extracted photoreceptor support base is next immobilized and dried naturally, and subsequently placed in an oven or the like to be completely dried. Since an electrophotographic photoreceptor having a photosensitive coating film with a uniform thickness is manufactured in a short period time, a quick-drying solvent is usually used as a solvent of the application liquid.
When using a quick-drying solvent, although a drying rate of the application liquid can be increased and the application liquid can be solidified in a short period time, since heat loss occurs after dipping at the coating film and the support base due to heat of vaporization as the solvent evaporates between the extraction and set to touch, an abrupt temperature drop occurs and the temperature of the coating film falls to or below dew point. When the temperature of the coating film drops to or below dew point, due to condensation of water vapor in the air, the coating film takes in moisture and causes the surface of the coating film to turn white (a blushing phenomenon). Whitening of the surface of the coating film as described above is not only unfavorable in terms of appearance but is also problematic in that the whitening significantly affects charging characteristics and abrasion resistance of the electrophotographic photoreceptor and lead to a fatal defect.
Although characteristics of laminated organic photo conductors are also affected by blushing, the impact on single-layer organic photo conductors is more prominent since the charge generating material exists on the surface of the photo conductor. As a result, an inconvenience in that various characteristics of the photo conductor such as repetition characteristics during continuous use, ozone resistance and abrasion resistance decline become pronounced.
In consideration of such circumstances, there are demands for suppressing blushing that occurs during production of positively-charged single-layer electrophotographic photoreceptors. Conventionally, a method of preventing the occurrence of blushing has involved bringing a holding member that is used during coating into contact with an inner surface of a support base and adjusting a length and material of the holding member to control a temperature of the support base. However, this method is not sufficient. Furthermore, while attempts have been made involving heating a support base during drying of a coating film (JP Patent Publication 63-77060), managing temperature of an application liquid (JP Patent Publication 6-222576), managing a difference in temperature between a coating atmosphere and an application liquid (JP Patent Publication 7-155683), and controlling humidity of a coating atmosphere (JP Patent Publication 8-95265), applying these methods require investment in facilities.
In contrast, as a method of preventing blushing without the use of specialized equipment, a method is proposed in which a solvent used, density, specific heat, and thickness of support base material, and thickness of a formed photosensitive layer are controlled so as to satisfy specific conditions (JP Patent Publication 2001-175008).
In recent years, from the perspectives of downsizing, cost reduction, reduction in power consumption, and the like of electrophotographic apparatuses, reductions in size and weight of electrophotographic photoreceptors are desired. In addition, reductions in material cost and necessary drive power with respect to photosensitive layer support bases by further weight reduction are also desired. While a reduction in weight of a support base can be readily achieved by reducing wall thickness of the support base, this also causes a decline in heat capacity of the support base itself. Since a decline in heat capacity of the support base makes it easier for heat of vaporization due to evaporation of a solvent during coating of a photosensitive layer to cool the support base down to or below dew point, blushing is likely to occur.
Therefore, when a thin-walled support base is used, depending on a method of controlling a solvent used, density, specific heat, and thickness of support base material, and thickness of a formed photosensitive layer so as to satisfy specific conditions as described in JP Patent Publication 2001-175008, the occurrence of blushing cannot be prevented.
In addition, when the support base is coated with an application liquid for forming a photosensitive layer, a coating film at an end of the support base bulges due to the effect of surface tension and is likely to lose more heat of vaporization. Therefore, with an electrophotographic photoreceptor using a thin-walled support base, blushing is more likely to occur at the end. When forming a photosensitive layer by an immersion-removal method, since the effect of gravity comes into play in addition to the effect of surface tension, the coating film at a lower end of the support base is likely to become thicker. As a result, particularly at the lower end of the photoreceptor, blushing tends to occur prominently.
The present disclosure has been made in consideration of the circumstances described above, and an object thereof is to provide a positively-charged single-layer electrophotographic photoreceptor which comprises a blushing-free photosensitive layer on a thin-walled support base.
The present inventors have found that the occurrence of blushing can be prevented with a positively-charged photoreceptor that uses a tubular photosensitive layer support base with a wall thickness of 0.7 mm or less by having a cross-sectional shape of a support base end surface satisfy specific conditions in order to reduce a liquid sump that occurs in a lower portion of a tubular photosensitive layer support base when the tubular photosensitive layer support base is pulled out of a coating tank during coating. The present disclosure is based on these findings. More specifically, the present disclosure provides the following.